It's not about working on the beach, it's about being able to go to the beach to rest and having an amazing work place for the time you want to be effective.
Focus / Defocus are for me the best way to be effective.
You are talking about polyphasic sleep, which when you train and skill is a great tool to sleep less, but I think these guys are not talking about that, they talk about being "la tête dans le guidon" as we say in France => head in the handlebar litteraly, or "sous l'eau" => underwater.
It is a myth.
Startups are all about creativity.
If you don't sleep enough, you are not creative. PERIOD.